Signal coupling apparatus utilizing hybrid transformer

ABSTRACT

A hybrid transformer signal coupling apparatus is disclosed wherein one winding is a coaxial conductor having an outer conductor which provides electrostatic shielding between the center conductor and a second winding wound on a core of magnetic material. The hybrid transformer is used in a directional coupler for broadband signals such as television signals carried via a coaxial cable.

United States Patent n 1 Zelenz 1 Feb. 13, 1973 [54] SIGNAL COUPLING APPARATUS UTILIZING HYBRID TRANSFORMER [75] Inventor: Martin L. Zelenz, Seneca Falls,

[73] Assignee: G T E Sylvania, Incorporated 221 Filed: Aug. 19, 1971 21 App]. No.: 172,981

[52] US. Cl. ..333/I0, 333/11, 336/174, 336/175, 336/195 [51] Int. Cl. ..II0lp 5/14, H03h 7/48 [58] Field of Search ..333/10,11;336/175,176, 195, 336/174; 325/308; 179/82, 112

[56] 1 References Cited UNITED STATES PATENTS 2,808,566 10/1957 Douma ...333/10X 8/1962 Simons .4333 10 12/1970 Werlau ..333/l0 Primary ExaminerPaul L. Gensler AttorneyNorman J. O'Malley et al.

[57] ABSTRACT A hybrid transformer signal coupling apparatus is disclosed wherein one, winding is a coaxial conductor having an outer conductor which provides electrostatic shielding between the center conductor and a second winding wound on a core of magnetic material. The hybrid transformer is used in a directional coupler for broadband signals such as television signals carried via a coaxial cable. 1

5 Claims, 3 Drawing Figures l SIGNAL COUPLING APPARATUS UTILIZING HYBRID TRANSFORMER BACKGROUND OF THE INVENTION This invention relates to signal coupling apparatus and more specifically to a hybrid transformer for coupling a predetermined portion of a signal from a primary transmission path to a secondary transmission path.

Various types of signal coupling apparatus for coupling signals fromone transmission path to another transmission pathare knownin the prior art. Typically, such signalcouplers'are arrangedso that the coupling is directional, that is a wave traveling in one direction in the primary path is coupled to the secondary path, but a wave traveling in the opposite direction in the primary path is notcoupled to the secondary path.

Where the range of frequencies of the wave being coupled to the secondary, path is relatively small, looptype or two-hole directional couplers may provide satisfactory results. Wherethe range of frequencies is large, however, such directional couplers do not provide satisfactory results.

Another typetis'a stripline direction coupler, however, stripline types also have a limited operational bandwidth and thecoupling is frequency dependent so that uniform coupling over abroad bandwidth is difficult'or impossible to achieve. Also, the physical size necessary to achieve low'frequency: response limits the Directional couplers utilizing transformer type signal couplers arecommonlyused for'such applications as couplingsignalsfrom onetransmission path to another in community antenna television (CATV) systems, for example, to connect signals on a feeder or distribution line toa subscriberdropline.'Hybrid transformer type directional couplers provide satisfactory operation over a broader range.offfrequen'cies; however, as the number of television channelsorother signals carried by the system is increased, the frequency range necessarily increases. Leakage inductance and interwinding capacitanceof prior art hybrid transformers degrade the frequency response of the directional coupler at high frequencies. One effect is that the coupling changes and thedirectivity ofthe directional coupler decrease at high frequencies causing a tilted frequency response characteristic. Also, the impedance matching of the signal coupler to the transmission medium, e.g., coaxial cable, deteriorates with frequency. These and otheradvantages of prior art signal-couplerslimit the frequency bandwidth zof= such signal couplers or of directional couplers utilizing such signal couplers.

OBJECTS ANDSUMMARY OF THE INVENTION Accordingly, it is a primary=objectof this invention to obviate the disadvantagesof theprior art.

ltlis another objectof thisinvention to provide a novelsignal coupler withimproved frequency response characteristics.

It is a further object of this invention to provide a hovel signal coupler with improved high frequency performance.

It isa furtherobjectof this invention to provide a novel signal couplerwith improved performance over a greater-frequency range-orbandwidth.

LII

.use of stripline types .to relatively. high frequency ranges.

It is a further object of this invention to provide a novel signal coupler for use in a directional coupler with improved performance.

It is a still further object of this invention to provide an improved signal coupler which is of simple construction.

In one aspect of this invention, the above objects and advantages are achieved in a signal coupling apparatus for coupling a predetermined portion of a signal from a primary transmission path to a secondary transmission path which includes a segment of coaxial conductor and a winding disposed ona core of magnetic material. The segment of coaxial conductor includes a center conductor for coupling to the primary transmission path and a coaxial outer conductor of predetermined length and diameter surrounding the center conductor. The coaxial conductor is disposed in the core so that the-winding on the core is electromagnetically coupled to the center conductor. The winding on the core is adapted to be connected to the secondary transmission path. The outer conductor of the coaxial conductor is connected to a common conductor to provide electrostatic shielding between the winding on the core and the center conductor.

In a more limited aspect of this invention, the abovedescribed signal coupling apparatus is utilized in a directional coupler and the primary and secondary transmission paths include coaxial cable segments.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates a signal coupler in accordance with the preferred embodiment of the invention;

FIG. 2 is a schematic illustration of signal couplers in accordance with the preferred embodiment of the invention utilized in a directionalcoupler; and

FIG. 3 illustrates the physical arrangement of a directional coupler in a housing for use 'incommunity antenna television systems. e

DETAILED DESCRIPTION OF THE INVENTION Fora better understanding of the present invention,

together with other and further objects, advantages, and capabilities thereof, reference is made to the following disclosure and appended claims in connection with the above-described drawings.

FIG. 1 illustrates signal coupling apparatus in accordance with the preferred embodiment of the invention. The type of signalcoupler illustrated is generally known as a hybrid transformer. A core of magnetic materiall0 has a winding 11 toroidally wound thereon. A coaxial conductor segment 12 is disposed in core 10 and hence winding 11. Coaxial conductor 12 has a center conductor 13 and a coaxial outer conductor 14 with an insulating dielectric 15 disposed between center conductor 13 and outer conductor 14. Insulation 15 may be any suitable material 'such as polyethylene foam. I

In operation, center conductor '13 iscoupled to the primary transmission path while winding 11 is coupled to the secondary transmission: path. Signals coupled through center conductor l3.are electromagnetically coupled to winding 11 thereby inducing a signal in winding 11 which is a portion or fraction of the signal carried by center conductor 13. The coupling between winding '11 and center conductor 13 depends upon the length of core and the number of turns of winding 11.

The signal coupler of FIG. 1 is particularly advantageous in systems utilizing coaxial cable for the primary transmission path because superior impedance matching is obtained between the impedance of the coaxial cable and center conductor 13 thereby reducing losses and reflections due to impedance mismatch. This advantage is particularly pronounced at high frequencies and tends to improve the frequency response characteristic of the signal coupler at high frequencies thereby extending the bandwidth or frequency range of the signal coupler.

One end of outer conductor 14 is preferably grounded to provide electrostatic shielding between center conductor 13 and winding 11. Electrostatic or capacitivecoupling between the windings in prior art transformer-type signal couplers tends to degrade the high frequency performance especially when the signal couplers are used as directional couplers. Specifically, the coupling and the directivity decrease at high frequencies. The grounding of outer conductor 14 thus decreases electrostatic coupling between center conductor 13 and winding 11 thereby improving the high frequency performance of the signal coupler. The frequency response of the signal coupler is dependent on the length and diameter of the outer conductor 14 and on thelength and diameter of core 10. In one embodiment of this invention, a length to diameter ratio of core 10 of approximately one was found satisfactory when the inner diameter of core 10 and the diameter of outer conductor 14 wereas nearly equal as possible. The ratio of the diameter of the outer conductor 14 to the inner conductor 13 should be selected to obtain the impedance of the coaxial transmission-medium (cable).

It should be noted that the grounding of outer conductor l4 will not appreciably affect the electromagnetic coupling between center conductor 13 and winding 11. The grounded outer conductor 14 acts as an unloaded, i.e., open circuit, secondary winding of a transformer with respect to electromagnetic coupling when one end only is grounded. Outer conductor 14 also tends to make the electromagnetic coupling between center conductor 13 and the individual turns of winding 11 more uniform by reducing unwanted capacitive coupling.

In FIG. 2 the signal coupler of FIG. 1 is illustrated in a directional coupler configuration wherein two signal couplers or hybrid transformers are utilized. In the preferred form of directional coupler in accordance with the invention, both signal couplers are equivalent to the structure illustrated in FIG. 1. FIG. 3 illustrates the directional coupler of FIG. 2 housed in a suitable housing 20, part of which is cut away to illustrate the a various components therein. Housing is suitable for use in community antenna television (CATV) systems wherein it is suspended from a metal strand or cable or otherwise mounted by suitable fasteners.

The primary transmission path includes two segments of coaxial cable 21 and 22. Coaxial cables 21 and 22 include center conductors 23 and 24, which protrude through openings of housing 20 and are seized by suitable clamps 25 and 26, and coaxial outer conductors 27 and 28, which are secured to housing 20 by suitable couplings 29 and 30. A secondary transmission path, to which a portion of signals traveling in the primary path is coupled, includes a segment of coaxial cable similar to coaxial cables 21 and 22 and secured to housing 20 by a suitable clamp and coupling 31. The coaxial cable segment in the secondary path includes a center conductor 32 and a coaxial outer conductor 33. Preferably, outer conductors 27, 28, and 33 are connected to a common conductor illustrated as ground.

The electrical components in housing 20 are carried on a printed circuit board 34 which is fastened in housing 20 by suitable screws 35. Center conductor 23 of coaxial cable 21 is electrically connected to a coupling capacitor 36 via a suitable contact pad 37 on printed circuit board 34. Capacitor 36 is connected to one end of a primary winding 40 of a hybrid transformer type signal coupler 41 in accordance with the invention. Primary winding 40 corresponds to center conductor 13 of FIG. 1. The other end of primary winding 40 is connected to a coupling capacitor 42 which is electrically connected via a suitable contact pad 43 on printed circuit board 34 to center conductor 24 of coaxial cable 22.

Transformer type signal coupler 41 includes a secondary winding 44 which corresponds to winding 11 of FIG. 1, a core 45 which corresponds to core 10, and an electrostatic shield 46 which corresponds to coaxial outer conductor 14. One end of shield 46 and one end of secondary winding 44 are connected to ground while the other end of winding 44 is connected to center conductor 32 of the coaxial cable in the secondary transmission path. In FIG. 3 the ground connections may be made to a ground plane on the under side of printed circuit board 34 by means of holes or contacts through printed circuit board 34.

The junction between capacitor 36 and primary winding 40 is connected to one end of a primary winding 50 of a hybrid transformer type signal coupler 51 in accordance with the invention. Primary winding 50 corresponds to winding 11 of FIG. 1. The other end of primary winding 50 is connected to ground. Transformer type signal coupler 51 includes a secondary winding 52 which corresponds to center conductor 13, a core 53 which corresponds to core 10, and an electrostatic shield 54 which corresponds to coaxial outer conductor 14. One end of shield 54 is connected to ground. Secondary winding 52 is connected between center conductor 32 and a junction 55 representing a contact pad on printed circuit board 34. Junction 55 is connected to ground by a resistor 56 in parallel with a capacitor 57.

A portion of a signal or wave in the primary transmission path traveling from center conductor 23 to center conductor 24 is coupled from primary winding 40 to secondary winding 44 of transformer 41 which induces a signal on center conductor 32 in the secondary transmission path. The same signal in the primary path is coupled across primary. winding 50 of transformer 51 and induces a signal across secondary winding 52 which is of an amplitude and phased such that the signal at junction 55 is zero in accordance with typical hybrid transformer directional coupler design. Thus, a portion of a wave traveling from center conductor 23 to center conductor 24 is coupled to center conductor 32.

A signal or wave traveling in the opposite direction in the primary path also causes signals to be induced across secondary windings 44 and 52, but the signal induced across secondary winding 44 is oppositely phased so that the two induced signals cancel at the junction with center conductor 32. A signal is induced at junction 55 which is absorbed by resistor 56 and capacitor 57. Preferably resistor 56 and capacitor 57 are of proper values such that no reflected wave occurs, that is, the impedance due to resistor 56 and capacitor 51 is equal to the characteristic impedance of the coaxial cable in the secondary path.

. In a directional coupler utilizing signal coupling apparatus in accordance with the invention, it was found that the through or insertion loss in the primary path was substantially decreased at high frequencies; the

I coupling (that is, the ratio of the power of a wave in duced in the secondary path to the power of a wave traveling in the forward direction in the primary path) was substantially improved at high frequencies and thus more uniform over the frequency range of interest; the directivity (that is, the ratio of the waves induced in the secondary path in response to waves traveling in opposite directions in the primary) was substantially increased at high frequencies, thus providing more uniform response over the frequency range of interest; and the return loss or signal reflection at all three coaxial cable terminations was substantially improved at high frequencies. Thus, a directional coupler in accordance with the invention provides improved operation over a greater range 'of frequencies than prior art devices. In one practical'embodiment of a directional coupler in accordance with the invention, satisfactory operation over a frequency range extending from less than mHz to at least 300 ml-lz was obtained which could have been obtained with prior art devices only with great difficulty and/or expense.

While there have been shown and described what are at present considered the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims.

[claim 1. Signal coupling apparatus for coupling a predetermined portion of a signal from a primary transmission path including first and second coaxial cable segments to a secondary transmission path including a third coaxial cable segment comprising:

a transformer including a core of magnetic material having a winding disposed thereon and a segment of coaxial conductor having a center conductor and a coaxial outer conductor surrounding said center conductor, said coaxial conductor being disposed in said core whereby said winding is electromagnetically coupled to said center conductor;

means for connecting said center conductor between said first and second coaxial cable segments;

means for connecting said winding between said third coaxial cable segment and circuit ground;

and

means connecting said outer conductor to circuit ground for providing electrostatic shielding between saidwinding and said center conductor, 2. Signal coupling apparatus as defined in claim 1 intransformer is a second segment of coaxial conductor having a center conductor and a coaxial outer conductor surrounding said center conductor, said coaxial conductor being disposed in said core whereby said secondary winding is electromagnetically coupled to said primary winding, said outer conductor of said second segment of said coaxial conductor being connected to circuit ground to provide electrostatic shielding between said primary winding and said secondary winding of said second transformer.

4. Signal coupling apparatus for coupling a predetermined portion of a signal from a primary transmission path including first and second coaxial cable segments to a secondary transmission path including a third coaxial cable segment comprising:

a first transformer including a core of magnetic material having a winding disposed thereon and a segment of coaxial conductor disposed in said core, said coaxial conductor having a center conductor connected between said first and second coaxial cable segments and electromagnetically coupled to said winding and a coaxial outer conductor surrounding said center conductor and connected to circuit ground for providing electrostatic shielding between said winding and said center conductor;

means connecting said winding between said third coaxial conductor and circuit ground; and

a second transformer having a primary winding connected between said first coaxial cable segment and circuit ground and a secondary winding connected between said third coaxial cable segment and an impedance whereby the signal coupling apparatus is sensitive to the direction of signal flow through said primary transmission path.

5. Signal coupling apparatus as defined in claim 4 wherein said primary winding of said second transformer is a toroidal winding on a core of magnetic material and said secondary winding of said second transformer is a second segment of coaxial conductor disposed in said core of said second transformer, said second segment of coaxial conductor having a center conductor connected between said third coaxial cable segment and said impedance and electromagnetically coupled to said primary winding and a coaxial outer conductor surrounding said center conductor and connected to circuit ground for providing electrostatic shielding between said winding and said center conductor of said second transformer.

i l 0' i Po-ww UNITED STATES PATENT omen CER'NWQATE v @F QQRREL HfiN Pasent'Ndh 3,716,806 Dated February 13, 1973 Im zentofls) Martin L. Zelenz It is certified that error appears in the above-identified patent and that saidLetters Patent are hereby corrected as shown below:

T" 1 .s i

Col. 1, line 53 of the specification Y'advantages" should read "disadvantages".

Signed and sealed this 3rd day of July 19 75.

'(SE J Attest:

I EDWARD MFLETCHERQJR. I Rene Tegtineyer Attesting Officer Acting Commlssloner of Patents P0405) UNITED STATES PATENT OFFICE CERTIFICATE 0F CORREL HON Patenf-Nbp 3,716,806 bated February 13, 1973 In'trentor(s) Martin L. Zelenz 7 It is certified that error appears in the above-identified patent and that saidLetters Patent are hereby corrected as shown below:

C01. 1, line 53 of the specification ,"advantages" should read "disadvantages".

Signed and Sealed this 3rd day of July 19,75.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. Rene Tegtineyer Attesting Officer A ti commlssloner of Patents P0405) UNITED STATES PATENT @FHQE QERTHFEQATE GE (IQREQHQN Patent b 3 716 ,806 Dated February 13 1973 Inventor(s) Martin L. Zelenz It is certified that error appears in the above-identified patent and that 'saidLetters Patent are hereby corrected as shown below:

Col. 1 line 53 of the specification ."advan'tages" should read "disadvantages".

Signed and sealed this 3rd day of July 1975.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. Rene Tegtmeyer Attescing Officer Acting Commissioner of Patents 

1. Signal coupling apparatus for coupling a predetermined portion of a signal from a primary transmission path including first and second coaxial cable segments to a secondary transmission path including a third coaxial cable segment comprising: a transformer including a core of magnetic material having a winding disposed thereon and a segment of coaxial conductor having a center conductor and a coaxial outer conductor surrounding said center conductor, said coaxial conductor being disposed in said core whereby said winding is electromagnetically coupled to said center conductor; means for connecting said center conductor between said first and second coaxial cable segments; means for connecting said winding between said third coaxial cable segment and circuit ground; and means connecting said outer conductor to circuit ground for providing electrostatic shielding between said winding and said center conductor.
 1. Signal coupling apparatus for coupling a predetermined portion of a signal from a primary transmission path including first and second coaxial cable segments to a secondary transmission path including a third coaxial cable segment comprising: a transformer including a core of magnetic material having a winding disposed thereon and a segment of coaxial conductor having a center conductor and a coaxial outer conductor surrounding said center conductor, said coaxial conductor being disposed in said core whereby said winding is electromagnetically coupled to said center conductor; means for connecting said center conductor between said first and second coaxial cable segments; means for connecting said winding between said third coaxial cable segment and circuit ground; and means connecting said outer conductor to circuit ground for providing electrostatic shielding between said winding and said center conductor.
 2. Signal coupling apparatus as defined in claim 1 including a second transformer having a primary winding connected between one of said first and second coaxial cable segments and circuit ground and a secondary winding connected between said third coaxial cable segment and an impedance whereby the signal coupling apparatus is sensitive to the direction of signal flow through said primary transmission path.
 3. Signal coupling apparatus as defined in claim 2 wherein said primary winding of said second transformer is a toroidal winding on a core of magnetic material and said secondary winding of said second transformer is a second segment of coaxial conductor having a center conductor and a coaxial outer conductor surrounding said center conductor, said coaxial conductor being disposed in said core whereby said secondary winding is electromagnetically coupled to said primary winding, said outer conductor of said second segment of said coaxial conductor being connected to circuit ground to provide electrostatic shielding between said primary winding and said secondary winding of said second transformer.
 4. Signal coupling apparatus for coupling a predetermined portion of a signal from a primary transmission path including first and second coaxial cable segments to a secondary transmission path including a third coaxial cable segment comprising: a first transformer including a core of magnetic material having a winding disposed thereon and a segment of coaxial conductor disposed in said core, said coaxial conductor having a center conductor connected between said first and second coaxial cable segments and electromagnetically coupled to said winding and a coaxial outer conductor surrounding said center conductor and connected to circuit ground for providing electrostatic shielding between said winding and said center conductor; means connecting said winding between said third coaxial conductor and circuit ground; and a second transformer having a primary winding connected between said first coaxial cable segment and circuit ground and a secondary winding connected between said third coaxial cable segment and an impedance whereby the signal coupling apparatus is sensitive to the direction of signal flow through said primary transmission path. 